As known, an electronic device or an operation device is usually equipped with an operation interface or a key control panel. Recently, in order to integrate the operation interface or the key control panel with the device, prevent damage and achieve the decorative benefit, the operation interface or the key control panel is designed toward miniaturization. Although the use of conventional printed circuit board (PCB) can function well, there are still some drawbacks. For example, the rigidity, hardness and thickness of the printed circuit board are relatively higher. Consequently, the use of the printed circuit board cannot comply with the miniaturization requirement. Moreover, the printed circuit board cannot be applied to various curvy surfaces. For solving the above drawbacks, a membrane circuit structure or a membrane switch structure has been widely used in the industry.
Nowadays, various membrane technologies have been disclosed. In accordance with one of the widely-used membrane technologies, a substrate of a membrane circuit board is made of polyester material such as polyethylene terephthalate (PET). Due to the insulating property, the heat resistance, the flexibility and the high resilience, the membrane circuit board formed of the PET membrane is softly or flexibly bendable. That is, the characteristics of the membrane circuit board are similar to the well-known flexible printed circuit (FPC). Moreover, after the polyester substrate of the membrane circuit board is printed with conductive silver paste, a designated circuit pattern is formed. The membrane circuit board is electrically conducted through the circuit pattern.
Depending on the applied devices, the operation interfaces or the key control panels with the membrane switch structure have many types. For example, a keyboard is equipped with a great number of keys. Consequently, the inner circuitries are very complicated. Generally, the layout area of the substrate is limited. For connecting the circuit pattern to the designated position, the path of the trace layout is usually very long. In addition to the keys, the keyboard also comprises luminous indication elements or backlight elements according to the practical requirements.
In case that an upper circuit layer and a lower circuit layer of the membrane circuit board are connected with each other in a bridging connection manner, the more complicated circuit structure can be obtained. Likewise, the upper circuit layer and the lower circuit layer are printed with conductive silver paste. Moreover, an insulating separation layer is arranged between the upper circuit layer and the lower circuit layer for separating the upper circuit layer and the lower circuit layer. The insulating separation layer has a perforation. A jumper is penetrated through the perforation to electrically connect the upper circuit layer and the lower circuit layer. Moreover, the membrane switch structure further comprises a panel layer and an adhesive layer. The panel layer is used as the operation panel. The adhesive layer is used for combining associated layers together. In other words, the membrane circuit structure is a multi-layered flat circuit board that is composed of elastic conductive material and insulating material.
Generally, as the length of the trace layout is increased, the impedance value is increased and the performance of the applied device is deteriorated. For example, in case that the key is located at a farther position, the response time of depressing the key is longer. Especially when the key is located at the bridging joint (i.e., the jumper), the impedance difference becomes larger. In case that a LED unit is located at the distal position of a LED strip, the power and brightness are lower than the LED units at other positions.
Conventionally, the impedance value of the circuit pattern is adjusted according to the thickness of the circuit pattern, the composition of the silver paste and silver fraction of the silver paste. However, the efficacy of adjusting the impedance value by the conventional method is limited. For example, the impedance value of the LED strip is adjusted to be about 120±30 ohms (a). However, the reduction of the impedance value is still unsatisfied.